Founder conflict

In my first year, when I onboarded a third cofounder after we passed the honeymoon phase, things began to crack. (For reference, that's about 3 months.) What opened my eyes was when we took a personality test and realized we were both terrible at operations. The test results inevitably broke the relationship, and we parted ways. What was the most uncomfortable conversation you've ever had with a cofounder? At what point did it happen in the startup? How did you resolve it, and what will you do in the future?